Field of the Invention
The present invention relates to a process for estimating the torque delivered by an internal combustion engine, and more particularly to a process of this type that works based on the observation of variations in the rotation speed of the crankshaft of the engine.
The torque delivered by an internal combustion engine is an important parameter for characterizing the working condition and the performance of an engine of this type, particularly when the latter is used to power an automobile. Devices known as "torque meters" make it possible to measure the torque delivered by an engine during bench testing. It is practically impossible to install such a device in an automobile, specifically for reasons of cost. Various processes have also been proposed for estimating torque from measurements of the instantaneous engine speed delivered by a sensor that is commonly installed today in automobiles in which the engine is controlled by a computer supplied with various signals, including the one supplied by the speed sensor. For reference, it is possible to consult the 1989 publication by W. B. RIBBENS and G. RIZZONI entitled "Applications of Precise Crankshaft Position Measurements for Engine Testing, Control and Diagnosis," publication number SAE 890885 of the "Society of Automotive Engineers" of the USA. This publication describes a process comprised of performing matrix algebra on instantaneous speed measurements taken during each engine cycle, in order to deduce from the latter the absolute value of the torque for each cylinder and thus to detect any abnormal variations in this torque. These calculations are complex and require powerful computing means which are not currently possible to install in an automobile. Moreover, according to this publication, it appears that this process is only usable at low speeds (less than 3500 rpm). Other torque estimating processes are based on an observation of variations in the speed of the engine within a time "horizon" that covers several successive fuel/air mixture combustion phases in several cylinders of the engine. An abnormal combustion, such as a misfire for example, in one of the cylinders being observed, substantially disturbs the global observation made and hence the estimate obtained from it.
Also known from European patent application 532 419, filed in the name of Regie Nationale des Usines Renault SA, are a process and a device for producing a value representative of the average "gas" torque produced by each combustion of a mixture in the cylinders of an internal combustion engine, the process being comprised of generating a "primary" value representing the duration of the passage in front of a sensor of each of a plurality of reference marks disposed on a ring integral with the flywheel or the crankshaft of the engine, of operating on the primary value to produce two secondary values, respectively representing the average angular speed of the reference marks during a combustion period in the engine and the projection onto a phase reference line of the marks related to the angular combustion periods, and the alternating component of the instantaneous angular speed of the marks, and of combining these two secondary values to produce a value representing the torque sought.
This process is capable of providing a satisfactory result insofar as 1) the "combustion center", or maximum pressure in a cylinder during a combustion, remains substantially set at a fixed angular position of the crankshaft and 2) the average speed of the engine does not vary substantially in the interval between two top dead centers.
The sole figure of the appended drawing represents graphs of the variations of the pressure P in a cylinder in which a combustion occurs, and the corresponding variations in angular speed .OMEGA., between two top dead centers reached consecutively by two pistons running in two cylinders of, for example, a four-cylinder engine. Experimentally, it is observed that at high speeds, with a fuel/air mixture of normal richness, the center of pressure P.sub.0 max remains centered on a position that is fixed relative to the top dead center, located in the first quarter of the angular interval separating two consecutive top dead centers. The corresponding fluctuations of the angular speed of the engine are represented in .OMEGA..sub.0 and have a maximum .OMEGA..sub.0 max located substantially at the end of the second quarter of the interval between top dead centers.
On the other hand, in other operating modes of the engine, For example at low speed or average speed with a lean fuel/air mixture, the pressure peak can be offset by up to a 45.degree. angle of rotation of the crankshaft (position P.sub.max) relative to the center of pressure P.sub.0 max. A corresponding phase shift is observed in the corresponding angular speed .OMEGA..sub.90, which in this case is phase-shifted by .pi./2 relative to the substantially sinusoidal fluctuations of the speed .OMEGA..sub.0, at the frequency of the top dead centers.
Outside these operating modes, an intermediate phase shift .DELTA..PHI. of .OMEGA. relative to .OMEGA..sub.0 of between 0 and .pi./2 is observed.
It is understood that an algorithm for calculating an estimate of the torque of an internal combustion engine which does not take into account this displacement of the peak or maximum angular speed of the crankshaft, cannot provide a satisfactory estimate of this torque in all of the possible operating modes of the engine.